Polyacetal polymers, which are commonly referred to as polyoxymethylene polymers, have become established as exceptionally useful engineering materials in a variety of applications. For instance, because polyoxymethylene polymers have excellent mechanical properties, fatigue resistance, abrasion resistance, chemical resistance, and moldability, they are widely used in constructing polymer articles, such as articles for use in the automotive industry and the electrical industry.
The mechanical properties of polyoxymethylene molding compositions are one of the reasons for their use in numerous applications. To improve their properties, polyoxymethylene polymers are often provided with additives to adapt the properties for a specific application, for example by using reinforcing fibers or tribological modifiers. For instance, polyoxymethylene polymers have been combined with a tribological modifier for producing polymer compositions well suited for use in tribological applications where the polymer article is in moving contact with other articles, such as metal articles, plastic articles, and the like. These tribological applications can include embodiments where the polymer composition is formed into gear wheels, pulleys, sliding elements, and the like. The addition of a tribological modifier can provide a composition with a reduced coefficient of friction, little frictional noise, and low wear.
In the past, high molecular weight polyolefins have been used to improve the wear resistance of polyoxymethylene resins. For instance, U.S. Pat. No. 5,482,987, which is incorporated herein by reference in its entirety, discloses a self-lubricating, low wear composition containing a polyoxymethylene and a lubricating system comprising a high molecular weight polyethylene, a high density polyethylene, and other components. U.S. Pat. No. 5,641,824, which is incorporated herein by reference in its entirety, discloses a self-lubricating melt blend of a polyoxymethylene and an ultra-high molecular weight polyethylene. However, polyoxymethylene compositions modified with these high molecular weight polyethylene polymers may have a less than desirable surface appearance as well as defects that may detract from the wear properties of the compositions and articles produced therefrom.
Although polyoxymethylene polymers have been tribologically modified in the past, further improvements are still necessary. For instance, a need exists for providing a polyoxymethylene polymer composition and a polymer article produced therefrom with improved tribological properties. In particular, a need exists for providing a polyoxymethylene polymer composition and a polymer article produced therefrom with a reduced coefficient of friction when in contact with other moving articles and improved wear properties.